1. Field of the Invention
This invention relates to zoom viewfinders having a zoom ratio of about 2.
2. Description of the Prior Art
U.S. Pat. Nos. 4,725,130 and 4,854,680 each describe a 3-unit Galilean-type zoom viewfinder. The finders disclosed in the above-mentioned patents all have a minus-plus-minus configuration. In these finders, the positive lens unit always moves for zooming, while the two negative lens units are stationary. U.S. Pat. No. 4,725,130 also discloses that the first negative lens unit may also move for zooming.
In the disclosed embodiments in both of these patents, the positive lens unit is disclosed to comprise two positive biconvex lens components located nearly in contact with each other. Although FIG. 5 of U.S. Pat. No. 4,854,680 shows that one of the components is a cemented doublet, this lens component itself is still positive and biconvex. The airspace between the two components of the positive lens unit is either fixed as shown in U.S. Pat. No. 4,725,130 or changes as shown in U.S. Pat. No. 4,854,680, but it is always held very small. That is, the lens components of the positive lens unit are nearly in contact with each other.
The inherent problem with this approach is that when the above minus-plus-minus component configuration is used and only the single positive unit is moved for zooming, the zoom finder suffers from defocusing, i.e. the image plane of focus of the lens moves. To accommodate this defocus, the lens of the observer's eye has to constantly adjust itself as the viewfinder zooms. Although, as disclosed in U.S. Pat. No. 4,725,130, the movement of the first lens unit in conjunction with the second lens unit can rectify this problem, a design modification such as this would loose an advantage of having fixed (i.e. stationary) outer lens elements.